Methods of desulphurizing iron and steel and gases, such as stack gases and the like

ABSTRACT

A method for desulphurizing iron, steel, stack gases and the like is provided in which rare earth oxides are reacted, in the presence of an agent, such as carbon, vacuum, reducing gases, etc. for reducing the oxygen level, with the sulphur to be removed to form one of the group consisting of rare earth sulphides, rare earth oxysulphides and mixtures thereof.

This invention relates to methods of desulphurizing iron and steel andthe like and particularly to a method of external desulpherizing ironand steel, stack gases, coal gases and the like using rare earth oxides.

External desulphurization of molten iron and steel has been practicedfor quite some time. It is a recognized, even necessary practice, inmuch of the iron and steel produced today. In current practices fordesulphurization magnesium metal, mag-coke, calcium oxide, calciumcarbide or mixtures of calcium oxide and calcium carbide are generallyused. Unfortunately, there are serious problems, as well as major costitems involved, in the use of all of these materials fordesulphurization. Obviously, both CaO and CaC₂ must be stored under dryconditions, since CaO will hydrate and CaC₂ will liberate acetylene oncontact with moisture. Magnesium is, of course, highly incendiary andmust be carefully stored and handled. There are also further problemsassociated with the disposal of spent desulphurization slags containingunreacted CaC₂.

We have found that these storage, material handling and disposalproblems are markedly reduced by using rare earth oxides in a low oxygencontent bath of molten iron or steel. The process is adapted to thedesulphurization of pig iron or steel where carbon monoxide, evolved bythe reaction, where carbon is used as a deoxidizer, is diluted with aninert gas such as nitrogen or by vacuum degassing the melt in order toincrease the efficiency of the reaction by reducing the likelihood offorming oxysulfides. The principle may also be used for desulphurizingstack gases from boilers, etc.

We provide a method of desulphurizing molten iron and steel as well asstack gases and the like by the steps of reacting rare earth oxide inthe presence of a deoxidizing agent with the sulphur to be removed toform one of the group consisting of rare earth sulphide and rare earthoxysulphide and mixtures thereof.

Preferably, hot metal is treated in a ladle or transfer car with rareearth oxides, by the simple addition and mixing of the rare earthoxides, by an injection technique in which the rare earth oxides areinjected into the molten bath in a carrier gas such as argon or nitrogenor by the use of an "active lining" i.e., a rare earth oxide lining inthe vessel. In any case, the chemical reactions involved are:

    2CeO.sub.2(s) + [C] = Ce.sub.2 O.sub.3(s) + CO.sub.(g)     ( 1)

    RE.sub.2 O.sub.3(s) + [C] + [S].sub.1w/o = RE.sub.2 O.sub.2 S.sub.(s) + CO.sub.(g)                                                ( 2)

And

    RE.sub.2 O.sub.2 S.sub.(s) + 2[C] + 2[S].sub.1w/o = RE.sub.2 S.sub.3(s) + 2CO.sub.(g)                                               ( 3)

The product sulphide or oxysulphide will be fixed in an `active` liningor removed by flotation and absorbed into the slag cover and vessellining depending upon the process used for introducing the rare earthoxide.

The products of desulphurization of carbon saturated iron with RE oxidesis dependent on the partial pressure of CO, pCO, and the Henrian sulphuractivity in the metal, h_(S). Using cerium as the representative rareearth, the following standard free energy changes the equilibriumconstants at 1500° C for different desulphurization reactions can becalculated from thermodynamic data in the literature:

    __________________________________________________________________________         REACTION                ΔG° cal.                                                                   K.sub.1773                             __________________________________________________________________________    2CeO.sub.2(s) + [C] = Ce.sub.2 O.sub.3(s) + CO.sub.(g)                                                     66000 - 53.16T                                                                          pCO = 3041                             Ce.sub.2 O.sub.3(s) + [C]+ [S].sub.1w/o = Ce.sub.2 O.sub.2 S.sub.(s) +        CO.sub.(g)                   18220 -26.43 T                                                                          pCO/h.sub.S = 3395                     Ce.sub.2 O.sub.2 S.sub.(s) + 2[C] + 2[S].sub.1w/o = Ce.sub.2 S.sub.3(s) +     2CO.sub.(g)                  66180 - 39.86T                                                                          p.sup.2 CO/h.sub.S.sup.2 = 3.6         3/2 Ce.sub.2 O.sub.2 S.sub.(s) + 3[C] + 5/2[S].sub.1w/o = Ce.sub.3            S.sub.4(s) + 3CO.sub.(g)     127050 - 72.1T                                                                          p.sup.3 CO/h.sub.s.sup.5/2 = 1.25      Ce.sub.2 O.sub.2 S.sub.(s) + 2[C] + [S].sub.1w/o = 2CeS.sub.(s) +             2CO.sub.(g)                  120,860  - 61.0T                                                                        p.sup.2 Co/h.sub.S = .027              C.sub.(s) + 1/2 O.sub.2(g) = CO.sub.(g)                                                                    -28200 - 20.16T                                                                         pCO/p.sup.1/2 O.sub.2 = 7.6                                                   × 10.sup.-7                      1/2S.sub.2(g) = [S].sub.1w/o -31520 + 5.27T                                                                          h.sub.S /p.sup.1/2 S.sub.2 = 5.4                                              × 10.sup.2                       __________________________________________________________________________

The thermodynamics of desulphurization with lanthanium oxide, La₂ O₃,are similar although, in this case, LaO₂ is unstable and there will beno conversion corresponding to CeO₂ → Ce₂ O₃.

In the foregoing general description of this invention, certain objects,purposes and advantages have been outlined. Other objects, purposes andadvantages of this invention will be apparent, however, from thefollowing description and the accompanying drawings in which:

FIG. 1 is a stability diagram showing w/o sulphur as partial pressure ofCO;

FIGS. 2a and 2b show Ce₂ S₃ and Ce₂ O₂ S layers on a pellet of CeO₂ ;

FIG. 3 is a graph of the theoretical CeO₂ required for removal of 0.01w/o S/THM;

FIG. 4 is a graph showing the volume of nitrogen required to produce agiven partial pressure of CO;

FIG. 5 is a graph showing the CeO₂ requirements as a function of partialpressure of CO; and

FIG. 6 is a stability diagram for stack gas systems treated according tothis invention.

Referring back to the discussion of free energy set out above, it isclear that these free energy changes may be used to determine the fieldsof stability of Ce₂ O₃, Ce₂ O₂ S, Ce₂ S₃, Ce₃ S₄ and CeS in terms of thepartial pressure of CO and the Henrian sulphur activity of the melt at1500° C. The resultant stability diagram is shown in FIG. 1, theboundaries between the phase fields being given by the followingrelationships:

    ______________________________________                                        BOUNDARY          EQUATION                                                    ______________________________________                                        Ce.sub.2 O.sub.3 - Ce.sub.2 O.sub.2 S                                                        log pCO = log h.sub.S + 3.53                                   Ce.sub.2 O.sub.2 S - Ce.sub.2 S.sub.3                                                        log pCO = log h.sub.S + 0.28                                   Ce.sub.2 O.sub.2 S - Ce.sub.3 S.sub.4                                                        log pCO = 0.83 log h.sub.S + 0.03                              Ce.sub.2 O.sub.2 S - CeS                                                                     log pCO = 0.5 log h.sub.S - 0.79                               Ce.sub.2 S.sub.3 - Ce.sub.3 S.sub.4                                                          log h.sub.S = -1.47                                            Ce.sub.3 S.sub.4 - CeS                                                                       log h.sub.S = -2.45                                            ______________________________________                                    

The phase fields in FIG. 1 are also shown in terms of the Henrianactivity of oxygen, h_(O), and the approximate [w/o S] in the iron meltusing an activity coefficient f_(S) ≈ 5.5 for graphite saturatedconditions.

The coordinates of the points B, C, D and E on the diagram are givenbelow:

    ______________________________________                                        COORDI-                                                                       NATES    B         C           D       E                                      ______________________________________                                        pCO atm. 9.8 ×0 10.sup.-3                                                                  6.5 × 10.sup.-2                                                                   1.0     1.0                                      h.sub.S  3.5 × 10.sup.-3                                                                   3.4 × 10.sup.-2                                                                   5.3 × 10.sup.-1                                                                 2.9 × 10.sup.-4                    Approx.                                                                       [w/o S]  6.4 × 10.sup.-4                                                                   6.2 × 10.sup.-3                                                                   9.6 × 10.sup.-2                                                                 5.3 × 10.sup.-5                    ______________________________________                                    

The points B and C represent simultaneous equilibria between theoxysulphide and two sulphides at 1500° C. These univariant points areonly a function of temperature. The points E and D represent the minimumsulphur contents or activities at which oxysulphide and Ce₂ S₃ can beformed, respectively, at pCO = 1 atm. Thus, carbon saturated hot metalcannot be desulphurized by oxysulphide formation below h_(S) ≈ 2.9 ×10⁻⁴ ([w/o S] ≈ 5.3 × 10⁻⁵) at pCO = 1 atm. However, lower sulphurlevels may be attained by reducing the partial pressure of CO.

The conversion of CeO₂ → Ce₂ O₃ → Ce₂ O₂ S → Ce₂ S₃ is illustrated inFIGS. 2a and 2b which show Ce₂ S₃ and Ce₂ O₂ S layers on a pellet ofCeO₂ (which first transformed to Ce₂ O₃) on immersion in graphitesaturated iron at ˜ 1600° C, initially containing 0.10 w/o S, for 10hours. The final sulphur content was ˜ 0.03 w/o S and the experiment wascarried out under argon, where pCO << 1 atm.

The conversion of the oxide to oxysulphide and sulphide is mass transfercontrolled and, as in conventional external desulphurization with CaC₂,vigorous stirring will be required for the simple addition process andcirculation of hot metal may be required in the `active` lining process.

From FIG. 1 it is apparent that the external desulphurization ofgraphite saturated iron is thermodynamically possible using RE oxides.For example the diagram indicates that hot metal sulphur levels of ˜ 0.5ppm (point E) can be achieved by cerium oxide addition even at pCO = 1atm. Desulphurization in this case will take place through thetransformation sequence CeO₂ → Ce₂ O₃ → Ce₂ O₂ S which required 2 molesof CeO₂ to remove 1 gm. atom of sulphur. The efficiency of sulphurremoval/lb CeO₂ added can however be greatly increased by the formationof sulphides. 1 mole CeO₂ is required per g. atom of sulphur for CeSformation and 2/3 moles CeO₂ for Ce₂ S₃ formation. The theoretical CeO₂requirements for the removal of 0.01 w/o S/THM for the variousdesulphurization products are given below and expressed graphically inFIG. 3.

    ______________________________________                                                 lb CeO.sub.2 /0.01      ft.sup.3 CO/0.01                             PRODUCT  w/o S.THM  ft.sup.3 CO/lb CeO.sub.2                                                                   w/o S.THM                                    ______________________________________                                        Ce.sub.2 O.sub.2 S                                                                     2.15       2.1          4.5                                          CeS      1.1        4.2          4.5                                          Ce.sub.3 S.sub.4                                                                       0.8        4.2          3.4                                          Ce.sub.2 S.sub.3                                                                       0.7        4.2          3.0                                          ______________________________________                                    

The volume of carbon monoxide produced in ft³ CO/lb CeO₂ and ft³ CO/0.01w/o S.THM are also given in the above table for each desulphurizationproduct. For efficient desulphurization the partial pressure of carbonmonoxide should be sufficiently low to avoid oxysulphide formation. Forexample, FIG. 1 shows that oxysulphide will not form in a graphitesaturated melt until [w/o S] < 0.01 when pCO ≈ 0.1 atm. It will formhowever when [w/o S] ≈ 0.10 at pCO = 1 atm. Thus by reducing the pCO inthe desulphurization process to 0.1 atm., hot metal can be desulphurizedto 0.01 w/o S with a CeO₂ addition of 0.72 lb/0.01 w/o S removed foreach ton hot metal.

The choice of the method of reducing the partial pressure of carbonmonoxide depends on economic and technical considerations. However, inan injection process calculations can be made for the volume ofinjection gas, say nitrogen, required to produce a given pCO. Thus:

    V.sub.N.sbsb.2 = V.sub.CO (1-pCO)/pCO

where

V_(CO) is the scf of CO formed/lb CeO₂ added

V_(N).sbsb.2 is the scf of N₂ required/lg CeO₂ added and

pCO is the desired partial pressure of CO in atm.

The results of these calculations for Ce₂ S₃ formation are shown in FIG.4, which also shows the [w/o S] in equilibrium with Ce₂ S₃(s) as afunction of pCO. From this figure is is apparent that the volume of N₂/lb CeO₂ required to form Ce₂ S₃ is excessive and if an injectionprocess were used a balance would have to be struck between sulphide andoxysulphide formation. When, for example, hot metal is to desulphurizedfrom 0.05 to 0.01 w/o S at pCO = 0.2 atm., ˜16 scf N₂ /lb CeO₂ would berequired for Ce₂ S₃ formation and the sulphur content would drop to 0.02w/o. The remaining 0.01 w/o S would be removed by oxysulphide formation.From FIG. 3, it can be seen that ˜2 lbs of CeO₂ /THM would be requiredfor Ce₂ S₃ formation and 2 lbs for Ce₂ O₂ S formation giving a totalrequirement of 4 lbs CeO₂ /THM.

Calculations similar to the one above have been used to construct FIG. 5where the CeO₂ requirements in lbs/THM are shown as a function of pCO.

When large volumes of nitrogen are used in an injection process the heatcarried away by the nitrogen, as sensible heat, is not large but theincreased losses by radiation may be excessive. Injection rates withCaC₂ for example are in order of 0.1 scf N₂ /lb CaC₂.

Vacuum processing is an alternative method of reducing the partialpressure of carbon monoxide. This is impractical in hot metal externaldesulphurization but not in steelmaking (see below).

Still another alternative approach to external desulphurization usingrare earth oxides is the use of active linings which would involve the`gunning` or flame-spraying of HM transfer car linings with rare earthoxides. Here the oxides would transform the oxysulphides during thetransfer of hot metal from the blast furnace to the steelmaking plant,an the oxide would be regenerated by atmospheric oxidation when the carwas emptied. It is estimated that for a 200 ton transfer car, conversionof a 2 mm layer (˜0.080 inch) of oxide to oxysulphide would reduce thesulphur content of the hot metal by ˜0.02 w/o S. This process has thefollowing advantages:

1. continuous regeneration of rare earth oxide by atmospheric oxidationwhen the car is empty,

2. reaction times would be in the order of hours,

3. the absence of a sulphur rich desulphurization slag,

4. the absence of suspended sulphides in the hot metal.

The mechanical integrity and the life of an "active" lining is, ofcourse, critical and some pollution problems may be associated withoxide regeneration by atmospheric oxidation.

With regard to steelmaking applications, vacuum desulphurization couldbe carried out by an "active" lining in the ASEA-SKF process andcirculation vacuum degassing processes.

In the case of desulphurization, assuming the following gas compositionat 1000° C:

    ______________________________________                                        Component       Vol. %                                                        ______________________________________                                        CO.sub.2        16                                                            CO              40                                                            H.sub.2         40                                                            N.sub.2         4                                                             H.sub.2 S       0.3                                                                           (200 grains/100 ft.sup.3.)                                    ______________________________________                                    

This equilibrium gas composition is represented by point A on thediagram illustrated as FIG. 6 where CO/CO₂ = 2.5 and H₂ /H₂ S = 133.This point lies within the Ce₂ O₂ S phase field and at constant CO/CO₂desulphurization with Ce₂ O₃ will take place up to point B. At point B,H₂ /H₂ S ≈ 10⁴ and the concentration of H₂ S is 0.004 vol.% (˜3grains/100 ft.³). Beyond this point, desulphurization is not possible.

In the foregoing specification, we have set out certain preferredpractices and embodiments of our invention, however, it will beunderstood that this invention may be otherwise embodied within thescope of the following claims.

We claim:
 1. A method of desulphurizing molten iron, steel, stack gasescontaining sulfur as an impurity comprising the steps of:a. reactingrare earth oxide in the presence of one of a separate deoxidizing agentand a deoxidizing atmosphere with sulphur to be removed to form one ofthe group consisting of rare earth sulphides and rare earth oxysulphidesand mixtures thereof, and b. removing said oxysulphides and sulphides.2. The method of desulphurizing molten iron, steel, stack gases and likematerials containing sulfur as an impurity as claimed in claim 1 whereinthe oxygen potential is maintained at a low level by reducing thepartial pressure of CO.
 3. The method of claim 2 wherein the partialpressure of CO is maintained below about 0.1 atmosphere.
 4. The methodof desulphurizing molten iron and steel as claimed in claim 1 whereinrare earth oxide is added to a molten bath of metal by injecting therare earth oxide beneath the surface of the molten bath in a stream ofinert gas sufficient to dilute carbon monoxide formed in the reaction toa level below about 0.1 atmosphere.
 5. The method of desulphurizingmolten iron and steel as claimed in claim 4 wherein the inert gas isnitrogen.
 6. The method of desulphurizing molten iron and steel asclaimed in claim 1 wherein rare earth oxide is added to a molten bath ofmetal subject to a vacuum sufficient to maintain the partial pressure ofcarbon monoxide below about 0.1 atmosphere.
 7. The method ofdesulphurizing molten iron and steel as claimed in claim 1 wherein themolten metal is poured into a vessel having a lining surface of rareearth oxides.
 8. The method of desulphurizing molten iron and steel asclaimed in claim 7 wherein the rare earth oxide lining is at least 2 mmin thickness.
 9. The method of desulphurizing molten iron and steel asclaimed in claim 7 wherein the vessel lining of rare earth isregenerated with oxygen after the desulphurized molten metal isdischarged prior to pouring another bath of molten metal into saidvessel.
 10. The method of desulphurizing molten iron and steel asclaimed in claim 7 wherein the vessel is subjected to a vacuumsufficient to maintain a partial pressure of carbon monoxide below 0.1atmosphere.